The conventional method for treating a sulphidic zinc concentrate is concentrate roasting, in which the sulphidic concentrate is roasted into zinc oxide and the iron in the concentrate forms chiefly zinc ferrite. Zinc oxide dissolves fairly easily, so that in the first stage the calcine is subjected to leaching, which is called neutral leaching. Zinc ferrite remains undissolved in neutral leaching and in order to recover this zinc from the ferrite a strong acid leaching is often used. Zinc ferrite residue also contains the ferric iron residue precipitated in neutral leaching. Ferric iron residue for its part contains in addition to ferric hydroxide co-precipitated aluminium hydroxide and rare metals, such as gallium and indium. The ferrite residue can also be fed into a Waelz kiln, in which the zinc is evaporated, and is then oxidised into zinc oxide and fed back into the leaching process. Waelz oxide can also be treated in a separate process step for the sake of recovering the other co-precipitated metals such as indium.
Nowadays the trend is more and more towards processes, in which at least part of the sulphidic zinc concentrate is fed into leaching without roasting. This enables the treatment of impure and fine-grained concentrates. A direct leaching process for zinc sulphide concentrate can be carried out in both atmospheric and pressure leaching processes. However, zinc sulphide leaching requires a far higher acid concentration than that used in the neutral leaching of a calcine, but because the fabrication of elemental zinc nearly always occurs electrolytically, the spent acid from electrolysis can be used in concentrate leaching. Zinc ferrite leaching requires the highest acid concentration of all. Sulphide concentrate leaching can be combined with a process in which the leaching of ferrites formed in roasting occurs as a strong acid leach and thus the leaching of ferrites is performed in connection with concentrate leaching. In that case a counter-current leaching process is used, where in addition to a strong acid leaching stage enabling zinc ferrite leaching there is also a weak acid leaching stage. A significant portion of concentrate leaching occurs for its part even in the weak acid leaching stage. These types of methods are described for instance in U.S. Pat. Nos. 6,475,450, 5,858,315 and 6,340,450 and in WO publication 2004/076698.
Zinc concentrate may contain rare metals such as indium and gallium, which it is desirable to recover. One possible method to implement the recovery of these metals is the processing of neutral leaching leachate in a Waelz kiln into Waelz oxide and the leaching of this oxide, whereupon the metals ending up in the oxide are made to return to the solution and are further recovered in liquid-liquid extraction. This kind of indium and gallium recovery in connection with a Waelz oxide leaching process is known in the prior art. This process is assisted by the fact that these metals have already enriched the Waelz oxide, because they are co-precipitated with ferric hydroxide in neutral leaching. In accordance with the method the zinc oxide that contains a valuable metal is leached by means of sulphuric acid, whereupon the metals and the zinc dissolve and the lead and silver plus other inert compounds in the oxide remain in the residue. The solution is routed to indium extraction, where indium is separated from the zinc, and the zinc sulphate solution is routed to the neutral leaching step. If the concentrate contains gallium, its recovery takes place in principle during indium recovery, whereupon indium and gallium are separated into their own phases.